1. Field of the Invention
The present invention relates to a radio communication system using a cellular phone such as a mobile phone and a car phone. More specifically, the invention relates to a code division multiple access system for eliminating a difference in output timing of data in each of a correlator of a pilot channel and that of an information channel using a queue for each spreading coefficient and a method for controlling transmission line correction timing of the system.
2. Description of the Related Art
Generally, CDMA (code division multiple access) includes a channel through which a pilot signal for making a transmission-line estimation flows (pilot channel) and a channel through which data flows (information channel). A receiving end can estimate a transmission-line response by transmitting a known pattern to the pilot channel. A received value of the information channel is corrected on the basis of the transmission-line estimated value, thereby decreasing an error rate of data.
FIG. 1 is a block diagram for explaining a prior art code division multiple access system and a method for controlling timing of transmission-line correction of the system. It shows a section 300 to which a transmission-line estimated value is applied for one finger. Information of an information channel is input to a correlator 311 via an input line 331. The correlator 311 correlates the information of the information channel with a spreading code and extracts data for each symbol (symbol information). When a plurality of fingers are used, the symbol information is input to a finger-to-finger delay adjustment section 312 in order to adjust a delay difference between fingers. When only one finger is used, no finger-to-finger delay adjustment is always required. The data of the finger-to-finger delay adjustment section 312 is supplied to a transmission-line correcting section 313.
On the other hand, information of the pilot channel is input to a correlator 321 via an input line 332. The correlator 321 correlates the information of the pilot channel with a spreading code and extracts data for each symbol (symbol information). When a plurality of fingers are used, the symbol information is input to a finger-to-finger delay adjustment section 322 in order to adjust a delay difference between fingers. When only one finger is used, no finger-to-finger delay adjustment is always required. The data of the section 322 is supplied to a transmission-line estimating section 323, in which a transmission-line response is obtained from the known pilot information. The section 323 outputs the transmission-line response to the transmission line correcting section 313 as transmission-line estimated value information.
The transmission-line correcting section 313 makes a transmission-line correction to the data of the information channel based on the transmission-line estimated value information obtained from the transmission-line estimating section 323. The corrected data is output from an output line 333.
There is a case where pilot and information channels differ in spreading coefficients in CDMA. For example, the spreading coefficient of the pilot channel tends to be set higher in order to decrease the error rate of the pilot channel. There is a case where the spreading coefficient of the information channel lowers in order to increase the transfer rate of data. Per-symbol time from when data is input to a correlator until it is output therefrom depends upon the length of a spreading code. The longer the spreading code (or the higher the spreading coefficient), the longer the per-symbol time.
Problems arising from variations in spreading coefficient will now be described in detail. FIG. 2A shows a relationship in symbol interval between data of a transmission line and that of a correlator when the spreading coefficient of each of pilot and information channels is 256, while FIG. 2B shows the same relationship when the spreading coefficient is 128. The correlators 311 and 321 observe a correlation between a spreading code and information of the information channel and a correlation between a spreading code and information of the pilot channel, respectively. Time required until data is output therefore corresponds to a value obtained by multiplying the spreading coefficient by per-chip time. The duration of the time is referred to as a one-symbol length in the spreading coefficient.
When the spreading coefficient is 256, symbol information items 411 to 414 in the transmission line correspond to data (symbol information) items 421 to 424 of the correlators 311 and 321 as shown in FIG. 2A. Paying attention to the symbol information item 411 with symbol number 0 in the transmission line, the symbol information item 421 is output from the correlators 311 and 321 after the symbol information item 411 is output, i.e., after the one-symbol length in the spreading coefficient. Time required until the symbol information items 412, 413 and 414 with symbol numbers 1, 2 and 3 are output from the correlators 311 and 321 as symbol information items 422, 423 and 424, respectively corresponds to the one-symbol length described above.
Similarly, when the spreading coefficient is 128, symbol information items 431 to 438 in the transmission line correspond to data items 441 to 448 of the correlators 311 and 321 as shown in FIG. 2B. Paying attention to the symbol information item 431 with symbol number 0 in the transmission line, the symbol information item 441 is output from the correlators 311 and 321 after the symbol information item 431 is output, i.e., after the one-symbol length in the spreading coefficient. Time required until the symbol information items 432, 433, 434, . . . and 438 with symbol numbers 1, 2, 3, . . . and 7 are output from the correlators 311 and 321 as symbol information items 442, 443, 444, . . . and 448 respectively corresponds to the one-symbol length corresponds to the one-symbol length described above.
The symbol length in the spreading coefficient of 256 is twice as long as that in the spreading coefficient of 128. Assume that symbol information item 411 with symbol number 0 in the spreading coefficient of 256 and the symbol information item in the spreading coefficient of 128 which is present in the transmission line during the same time period as the symbol information item 411, correspond to symbol information items 431 and 432 with symbol numbers 0 and 1, respectively. In this case, data of the correlators 311 and 321 corresponding to the symbol information item 411 with symbol number 0 in the spreading coefficient of 256 is symbol information item 421.
Data of the correlators corresponding to the symbol information items 431 and 432 with symbol numbers 0 and 1 in the spreading coefficient of 128 is symbol information items 441 and 442. The timing at which the symbol information items 411, 431 and 432 in the same transmission line are output from the correlators corresponds to the timing at which the symbol information items 421, 441 and 442 are output from the correlators. It is thus understood that the timing at which the symbol information item in the spreading coefficient of 256 is output from the correlators is delayed by a difference between the symbol length in the spreading coefficient of 256 and that in the spreading coefficient of 128.
When the pilot channel is transmitted with a spreading coefficient of 256 and the information channel is transmitted with a spreading coefficient of 128, the timings of a pilot symbol and an information symbol differ between the transmission line and the outputs of the correlators.
When the information channel does not change in spreading coefficient, a buffer is simply provided for an amount of delay to delay information of the information channel. The information of the information channel can thus be time-adjusted to that of the pilot channel that is transmitted during the same period in the transmission line. In the foregoing case, the information channel with a spreading coefficient of 128 is stored in the buffer and delayed by the time for 128 chips corresponding to a difference in symbol length. The symbol information items whose timing is the same as that of the pilot channel with a spreading coefficient of 256 in the transmission line can queue for each other.
When the information channel changes in spreading coefficient, an amount of delay varies dynamically and thus the control of timing becomes very complicated. This problem will be described with reference to FIGS. 3A and 3B.
FIGS. 3A and 3B show data of the correlators acquired when the information channel changes in spreading coefficient. In FIG. 3A, the spreading coefficient changes from 256 to 128. In FIG. 3B, it changes from 128 to 256.
First, let us consider a case where symbol information items 511 and 512 having symbol numbers 8 and 9 are transmitted with a spreading coefficient of 256 and symbol information items 513, 514, 515 and 516 having symbol numbers 0, 1, 2 and 3 are transmitted with a spreading coefficient of 128. The symbol information items 511 to 516 propagate through the transmission line at their respective symbol-length intervals. In contrast, symbol information items 521 to 526 output from the correlators propagate as shown in FIG. 3A. The symbol length of symbol information item 521 output from the correlators, which corresponds to the symbol information item 511 transmitted with a spreading coefficient of 256, is equal to the symbol length in the spreading coefficient of 256. However, the symbol interval of symbol information item 522 output from the correlators, which corresponds to the symbol information item 512 transmitted with a spreading coefficient of 256, is equal to the symbol length in the (changed) spreading coefficient of 128. The symbol information items 523 to 526 output from the correlators, which correspond to the symbol information items 513 to 516 transmitted after the spreading coefficient is changed, are output at symbol-length intervals in the spreading coefficient of 128.
Let us consider another case where symbol information items 531 and 534 having symbol numbers 16, 17, 18 and 19 are transmitted with a spreading coefficient of 128 and symbol information items 535 and 536 having symbol numbers 0 and 1 are transmitted with a spreading coefficient of 256. The symbol information items 531 to 536 propagate through the transmission line at their respective symbol-length intervals. Symbol information items 541 to 546 output from the correlators propagate as shown in FIG. 3B. The symbol interval of symbol information items 541 to 543 output from the correlators, which correspond to the symbol information items 531 to 533 transmitted with a spreading coefficient of 128, is equal to the symbol length in the spreading coefficient of 128. However, the symbol interval of symbol information item 544 output from the correlators, which corresponds to the symbol information item 534 transmitted with a spreading coefficient of 128, is equal to the symbol length in the (changed) spreading coefficient of 256. The symbol information items 545 and 546 output from the correlators, which correspond to the symbol information items 535 and 536 transmitted after the spreading coefficient is changed, are output at symbol-length intervals in the spreading coefficient of 256.
When the spreading coefficient is changed in the manner described above, the last symbol information before the change in spreading coefficient is used as the symbol length after that change.
Time required until the symbol information is output from the correlators also changes after the spread coefficient changes.
Assuming that the pilot channel is continuously transmitted with a fixed spreading coefficient, a time difference varies in units of symbol in a stage succeeding to the correlators of the pilot and information channels that are transmitted during the same period in the transmission line. In order to correctly make a transmission-line correction, however, symbol information items need to be used in combination of the information and pilot channels that are transmitted during the same period in the transmission line.
In order to adjust the time difference, timing needs to be controlled so as to change an amount of delay from the symbol information whose spreading coefficient varies. However, it is necessary to always recognize a boundary of symbol information items whose spreading coefficient vary, thus complicating the control of timing. Further, an amount of delay of different symbols needs to be changed according to a combination of spreading coefficients, which also complicates the control of timing.
The foregoing code division multiple access system has the following problem. When the spreading coefficient is changed during the transmission of symbol information in the transmission line, the symbol length of the symbol information is changed and the time required until the symbol information is output from the correlator is changed.
In order to resolve the above problem and exactly make a transmission-line correction, an amount of delay needs to be changed based on symbol information whose spreading coefficient varies and an amount of delay of different symbols needs to be changed according to a combination of spreading coefficients. The control of timing can thus be complicated.